From the past, considerable efforts have been made for high-speed silver halide photographic light-sensitive materials. It is known that a sensitizing dye for use in spectral sensitization has a great effect on the properties of a silver halide photographic light-sensitive material. Even a slight difference in structure of sensitizing dyes sometimes greatly affects photographic properties, such as sensitivity, fog, and storage stability. However, it is difficult to estimate photographic effects from sensitizing dyes' structures in advance. Therefore, many researchers have been making an effort to synthesize many sensitizing dyes and to investigate their photographic properties. Often used sensitizing dyes are those composed of a nitrogen-containing heterocycle having a sulfoalkyl group, as a partial structure. With respect to a sulfoalkyl group, a 2-sulfoethyl group, a 3-sulfopropyl group, a 4-sulfobutyl group, and a 3-sulfobutyl group are well known. However, the present state of the art is that other sulfoalkyl groups besides these groups have scarcely been investigated, so that it is impossible to estimate photographic properties that are shown by various kinds of other sulfoalkyl groups.
Further, in order to increase the sensitivity of a photographic light-sensitive material, reduction sensitization has long been investigated. For example, tin compounds in U.S. Pat. No. 2,487,850, polyamine compounds in U.S. Pat. No. 2,512,925, and thiourea dioxide-series compounds in British Patent No. 789,823, respectively, have been disclosed as useful reduction sensitizers. Further, in Photographic Science and Engineering, Vol. 23, page 113 (1979), properties of silver nuclei, which had been manufactured by various kinds of reduction sensitization methods, were investigated, and as a result it was found that methods for using dimethylamine borane, stannous chloride, or hydrazine, and methods for ripening at a high pH, or at a low pAg, are useful for reduction sensitization. Reduction sensitization methods are also disclosed in U.S. Pat. Nos. 2,518,698, 3,201,254, 3,411,917, 3,779,777, and 3,930,867. JP-B ("JP-B" means examined Japanese patent publication) Nos. 33572/1982 and 1410/1983 each describe not only selection of a reduction sensitizer but also how to perform reduction sensitization.
However, the present inventor's investigation found that, when spectral sensitization was conducted by adsorbing a sensitizing dye onto a reduction-sensitized silver halide grain, and particularly when spectral sensitization was performed in the spectral regions of green and red, it was extremely difficult to attain sufficient spectral sensitivity without causing some photographically adverse effects (e.g. an increase of fog).
Further, use is well known of a method in which a sensitizing dye is adsorbed onto a silver halide grain at a high temperature (not lower than 50.degree. C.), so that the sensitizing dye on the silver halide grain is prevented from desorption in a light-sensitive material (especially in an atmosphere of high humidity), or a method in which a sensitizing dye is adsorbed onto a silver halide grain before chemical sensitization, in order to increase sensitivity. However, it was found that when such methods were applied to a reduction-sensitized photographic emulsion, in which a spectral sensitizing dye, especially for a green and red spectral region, had been adsorbed on a silver halide grain, fog extremely increased.
Therefore, there is need for development of a technology by which reduction-sensitized silver halide grains are spectrally sensitized to attain high sensitivity without causing any adverse effects, such as fog formation.